The present invention relates to a steering wheel having an airbag device (an airbag module) for protecting the head and the part near the head of an occupant seated on the driver's seat of a vehicle, and to a method for assembling the steering wheel.
For example, Japanese Laid-Open Patent Publication No. 2004-256078 discloses a steering wheel with an airbag device that is attached to a steering wheel body fixed to a steering shaft, together with horn switch mechanisms. This type of airbag device has a bag accommodation space for accommodating a folded airbag between a pad having an ornamental surface of the steering wheel and a bag holder attached to the pad. A horn plate is arranged on a side of the bag holder that faces the steering wheel body. The horn plate is secured to a metal core of the steering wheel body. A plurality of horn switch mechanisms are provided between the horn plate and the bag holder. The horn switch mechanisms support the bag holder between the horn plate and the bag holder.
Each of the horn switch mechanisms includes a bolt, an urging member, a movable contact, a fixed contact. The bolt is passed through the horn plate and the bag holder from the steering wheel body. The bolt is then fastened with a nut from a side of the bag holder that faces the pad. The urging members urge the bag holder away from the horn plate, and support the entire airbag device through the bag holder. The movable contacts are provided in the vicinity of the bag holder, and the fixed contacts are provided in the vicinity of the pad. When the pad is pressed toward the steering wheel body, the movable contact of a horn switch mechanism contacts the corresponding fixed contact. This activates the horn of the vehicle. Further, when the vehicle receives impact, gas is generated in an inflator. The airbag breaks open the pad and is instantly inflated and deployed in the vicinity of the vehicle seat. This attenuates the impact received by the driver.
In the above described steering wheel, the airbag device is installed in the steering wheel body together with the horn switch mechanisms. Thus, when the airbag device is installed in the steering wheel body in a pressing manner, or when a user strikes hard against the airbag device, the horn switch mechanisms receive a great load. Therefore, in the above described airbag device, the horn switch mechanisms must be firmly secured, and for example, the horn switch mechanisms are fastened to the steering wheel body with bolts. In other words, the securing structure of horn mechanisms is limited.
In the above described steering wheel, the horn switch mechanisms are located outside the bag accommodation space when viewed from front. This increases the size of the airbag device. To solve this problem, the horn switch mechanisms may be arranged to overlap with the bag accommodation space when viewed from front. However, if this configuration is employed in the steering wheel disclosed in the above document, the distal ends of the bolts and nuts of the horn switch mechanisms largely project into the bag accommodation space. Thus, when the airbag is inflated, the bolt distal ends and angular parts of the nuts can damage the airbag. Therefore, in a conventional airbag device, the horn switch mechanism need to be located outside of the bag accommodation space so that the horn switch mechanism do not hinder inflation of the airbag.
Other than the above document, for example, Japanese Laid-Open Patent Publication No. 2004-284414 discloses a steering wheel with an airbag device. The airbag device includes a bag holder, which serves as a base. An airbag and an inflator for inflating and deploying the airbag are installed in the bag holder. Also, a pad is attached to the bag holder to cover the airbag. One end of each horn switch mechanism is connected to the bag holder. The other end of the horn switch mechanism is connected to a steering wheel body through a horn plate.
According to the structure of a horn switch mechanism disclosed in Japanese Laid-Open Patent Publication No. 2004-284414, coil spring units are placed between the bag holder and the horn plate. In this state, bolts are inserted to the coil spring units from a side of the horn plate that faces the steering wheel body. When the distal end of each bolt is passed through the coil spring unit and threaded to a seat of the bag holder. At the same time, the proximal end of the bolt is fixed to the horn plate, so that the horn mechanism is secured to the bag holder and the horn plate. On the other hand, the airbag and the pad are secured to a side of the bag holder opposite to the horn switch mechanism. Therefore, in the airbag device disclosed in Japanese Laid-Open Patent Publication No. 2004-284414, components other than the horn switch mechanisms are installed in accordance with the direction in which of the bolts of the horn switch mechanisms are assembled.
That is, a component that is at the lowermost position when viewed in the assembling direction of the bolts is the pad. Accordingly, the pad is first supported with its back side (side at which the airbag is accommodated and opposite side to the ornamental surface) facing upward. Then, while accommodating airbag in the pad, the bag holder is secured to the pad. Subsequently, horn switch mechanisms and a horn plate are secured to the bag holder. Then, a coil spring unit is arranged between the bag holder and the horn plate. The distal end of each bolt inserted in the horn plate is passed through the coil spring unit and threaded to the seat of the bag holder. This is how the proximal end of each bolt is fixed to the horn plate.
However, in this method, where the pad is supported while being installed, the manner in which the pad is supported needs to be changed for pads having different shapes. Since the pad has an ornamental surface, its shape varies in accordance with the type of car. Thus, when assembling the airbag device, a plurality of methods for supporting the pad need to be prepared in accordance with the type of car.